1. Field of the Invention
The present invention relates to an implantable heart defibrillator and to a defibrillation method.
2. Description of the Prior Art
It is known that cardiac fibrillation can be triggered by a low-voltage alternating current applied between the tip of an ordinary, implanted pacemaker electrode and an indifferent electrode in the body, e.g. the pacemaker enclosure. Thus, atrial fibrillation can be triggered by an alternating current, with a peak-to-peak amplitude of 18 V, applied via a 2 kohm series resistance to an electrode in the atrium. A lower series resistance could trigger ventricular fibrillation. Ventricular fibrillation can otherwise be induced by an alternating current with an amplitude of e.g. 18 volts peak-to-peak, applied, via a 0.5 kohm series resistance, to an electrode in the ventricle. For fibrillation to be induced, the alternating current must be applied for a number of time periods lasting 1 to 2 s with inter-period intervals of the same duration.
Once fibrillation has been triggered, it can, as is known, be terminated with a defibrillator supplying a brief, powerful discharge pulse with a voltage on the order of 1,000 V and an energy content of 4 to 40 J. When defibrillation is successful, i.e. fibrillation is terminated, the heart can then immediately resume normal rhythm, indicating that an optimum energy level was selected for the defibrillation pulse, e.g. 40 J applied externally. If cardiac arrest occurs, this indicates that more energy, e.g. 120 J applied externally, was used than was needed for restoration of normal heart rhythm. In the latter instance, additional defibrillation must be considered, however, the heart can often be induced to resume normal rhythm if subjected to conventional pacemaker stimulation at e.g. 70/min. In order to achieve the most rapid possible recovery of the heart, this stimulation should be terminated as soon as sinus rhythm is detected. Recovery is then very rapid, and the heart usually returns to its normal state after about 10 minutes.
The above-described mechanism for triggering heart fibrillation with the aid of a low-voltage alternating current can also be used for terminating fibrillation. One example thereof is the external AC defibrillators employed during the defibrillator's infancy. These employed an alternating current with several hundred volts and a frequency of 50-60 Hz. See Paul J. Troop, "Implantable Cardioverters and Defibrillators", Current Problems in Cardiology, Volume XIV, No. 12, Dec. 1989, pp. 729-731.
European Application 0 588 127 describes an implantable heart defibrillator which utilizes the heart's anisotropic properties for making cells, having a certain "preferred" orientation, refractory by means of preparatory, relatively low-energy pulses. Fibrillation can then be stopped with a subsequent defibrillation shock with less energy than would be required in conventional defibrillation. Defibrillation can sometimes be achieved solely with the preparatory, low-energy pulses without any subsequent defibrillation pulse. These preparatory, low-energy pulses are emitted for a period of time lasting from 10 ms up to the duration of a refractory period.
European Application 0 588 125 discloses heart defibrillation using defibrillation sequences consisting of low-energy stimulation pulses, and conventional defibrillation shocks. The low-energy stimulation pulses cause defibrillation of parts of the heart, mainly those regions around the electrode used, whereas conventional defibrillation shocks are required to defibrillate the rest of the heart. The amplitude and duration of the low-energy stimulation pulses used significantly exceed pacemaker pulses.